1. Field of the Invention
The present invention relates to a method and an apparatus for controlling a four-wheel drive system of a motor vehicle, and more particularly to such method and apparatus for controlling clutch means such as a wet-type multiple-disk clutch for controlled distribution of an engine torque to front and rear drive wheels of the vehicle.
2. Discussion of the Prior Art
An example of a full-time 4-wheel drive system for a motor vehicle is proposed in U.S. Pat. No. 4,702,341, in which the output shaft of an automatic power transmission is connected to a carrier of a planetary-gear type center differential device. A ring gear which is one of two output members of the center differential device is connected to a rear wheel drive shaft, while a sun gear which is the other output member of the differential device is connected to a front wheel drive shaft via transmission means which consists of sprockets and a chain belt. A differential control clutch is disposed so as to connect two members of the planetary carrier, sun gear and ring gear, such that the torque transmitted through the differential control clutch may be changed.
While the differential control clutch is in the released or fully disengaged position, the center differential device can fully perform its differential operation. While the differential control clutch is in the fully engaged position, the center differential device is placed in the disabled state wherein the rear drive shaft and the front drive shaft of the vehicle are directly connected. While the differential control clutch is controlled for partial engagement between its fully disengaged and fully engaged positions, the distribution of the engine torque to the rear and front drive shafts may be suitable adjusted. The above-identified patent also discloses a control device which controls the differential control clutch to regulate the transmission torque of the clutch, according to the input torque, such that the transmission torque of the clutch increases with an increase in the throttle opening angle of the engine (increase in the operating amount of the accelerator pedal of the vehicle), so that the differential function of the center differential device is limited as the throttle opening angle is increased. This arrangement assures high drivability and driving stability of the vehicle where a relatively high output of the engine is required.
As described above, the differential control clutch is provided to limit the differential operation or function of the center differential device of the 4-wheel drive system. The differential control clutch is fully engaged and fully disengaged when needed, and the amount of engagement of the clutch is adjusted when needed. Alternatively, the amount of engagement of the differential control clutch is continuously controlled in order to continuously limit the differential function of the center differential device in a controlled fashion. In the latter case, the force of engagement of the clutch is predetermined so that the desired distribution ratio of the torque (bias ratio) between the front and rear wheels may be achieved due to a controlled slip amount of the clutch. Where an electronic control device is used to control the differential control clutch, the control device is adapted to release the differential control clutch once after the vehicle is started, in order to determine the differential ratio of the center differential device, so that the differential control clutch is inhibited from limiting the differential function of the center differential device when the determined differential ratio exceeds a predetermined reference value, due to an excessive amount of difference in the diameter of the front and rear wheels, which difference is caused by a burst of the tire or by snow chains installed on the tires.
In the former method wherein the engagement force of the differential control clutch is maintained at the predetermined value, the clutch is not fully disengaged or released during running of the vehicle. In the latter method wherein the clutch is fully disengaged only once during running of the vehicle, the clutch is not released again as long as the differential ratio of the center differential device determined by the electronic control device is maintained below the predetermined reference value.
In the above-described method of controlling the differential control clutch, the operation of the center differential device is always limited in a controlled degree by a controlled amount of engagement or slip of the clutch, so that the ratio of torque distribution to the front and rear wheels (bias ratio) is controlled to a predetermined optimum value. In this arrangement, the differential control clutch is fully disengaged only when the engine is stopped or when the differential ratio of the center differential device is detected. Accordingly, the differential control clutch is almost always held in a partially engaging condition, namely, in a slipping condition, as long as the vehicle is running in a steady state at a relatively high speed. Where a wet-type multiple-disk clutch is used as the differential control clutch, the friction surfaces of the clutch tend to be insufficiently supplied with a lubricant. The insufficient lubricant supply causes reduced life expectancy of the clutch due to wear, increased friction coefficient of the friction surfaces, abnormally elevated operating temperature of the clutch due to insufficient cooling by the lubricant, and early loss of the friction characteristics of the clutch.